A corollary of this is, when you see an IR picture of a product with very different surfaces like rubber, shiny metal, matte paint, plastic, glass, etc. you know that almost for sure the measurement is unreliable because at most they calibrated the camera for the emissivity coefficient of one of the surfaces. And they vary quite a lot...
The one exception you have to be careful is with unpainted metal (high reflectivity). If polished it can be so low that even calibration doesn't help -- because you'd need extremely precise calibration that isn't practical and because reflections and noise/other heat sources will pollute you readings. In that case a simple sticker (or thermocouple reading) would be more adequate.
[0] https://www.raspberrypi.org/magpi/raspberry-pi-specs-benchma...
The four factors are Emissivity (ε), Absorptivity (α), reflectivity (ρ) and transmissivity (t). Emissivity is only one facet. Ideal is high Emissivity and low reflectivity.
Six years of grad school...
Cheers!
Whether the material is reflective or transmissive just means you'll be "seeing" IR sources reflected off the target or from behind the target, respectively. You do have to keep that in mind if there's an object not in ambient temperature in those locations (e.g. a clear sky is of very low temperature, a person is around 35C).
Ideally you'd want to coat any metal surface with black epoxy, and set the IR camera Emissivity Coefficient to 0.9 (the coefficient for black epoxy).
And your point here is completely understandable too - there is a kinda-heatsink there already. Just add a fan, and we're good.
My point is - seeing a large black hole where SoC goes (color corresponding to +20-ish degrees C by the chart) is utterly confusing, since SoC is the hottest point actually.
Yes environment typically will have some residual "noise" at those wavelengths, which you can check its intensity and spatial profile by taking a "dark frame" if you're in a strange environment and are really suspicious, but it's hardly going to alter what you're seeing in any qualitative way. Assuming someone isn't sending a focused beam of exactly that size at exactly that spot at an exactly correct angle at that particular wavelength.
A polished piece of metal makes a shitty black body. This is also why shiny metal (foil) is used to curb unwanted radiated heat transfer everywhere from thermos flasks and cryostats to space probes. (The lower emissivity further improves the efficiency of multi-layer insulation.)
Yes, reflectance of room temperature aluminum at those wavelengths is pretty good (not true for all metals BTW). Yes, this usually makes it hard to distinguish thermal radiation and reflected radiation with metals. What are you trying to say though? That whatever comes off from a metal must always be a reflection coming from somewhere else?
> Thus, their own Planck spectrum is (approximately) scaled down by their emissivity, and consequently the radiation in the measured MIR band is mostly what is reflected, which tends to come from the room-temperature environment.
I don't know what you mean by "Planck spectrum is (approximately) scaled down" (as "Planck spectrum" only refers to thermal radiation and is generated in a separate process from reflected photons [one is governed by the conduction band whereas the other is governed by everything up to Fermi level] and you can't hope to suppress thermal radiation by simply shining random environmental light on a metal --there is no such thing as "scaling down" of thermal radiation unless you engineer such property), but there is just no way that 10 micron photons at that intensity could be coming from a room-temperature environment.
So your blanket statements about metals aside, the hot area in that picture is due to a very specific signal which can't be due to something that's reflected from the environment. No significant fraction of those 10 micron photons coming off from that localized the area around the CPU could have originated from the environment --assuming that those pictures aren't taken in a hot oven and someone focused the thermal radiation on to the heatsink to get that amount of intensity.
And as I mentioned, that's pretty trivial to test. If those 10 micron photons were coming from the environment as you or the parent comment suggest, the thermal camera would report ~60C even when you look at Pi 4 when it is cooled (again, this is something can use as "dark frame" and subtract off from all readings if you're trying to be more accurate). This is clearly not the case, though, as you can see in the video on the blog post.
What are you trying to say though?
kees99 and klickverbot are saying it's unlikely the CPU is actually 23°C, especially given the author's statement the CPU was around 60°C, and that it's well known taking thermal camera images of things with different emissivities will produce inaccurate results.
kees99 is also saying, given that the thermal image doesn't accurately measure the temperature of the CPU, the article's statement that the metal casing helps isn't really warranted.
The CPU is the heat generator there, and is in contact with metallic regions around 60C (the red ring, if you compare to the real picture and follow the metallic bevels), where heat conductivity abruptly drops, which is what I've been talking about from the beginning. Since the heat is generated by the CPU and flows to the metal casing and to the PCB, the CPU can't be lower than 60C.
I agree that the reading for the inner region of the metal casing (which is not the CPU) must be off, and it's probably because the emission intensity there isn't strong enough and the camera software is mixing the emission and reflection when inferring the temperature (which gives physically incorrect results because the spectrum won't obey Planck's law, but the error depends on how different the temperatures are, and gets much stronger as they drift apart) rather than doing something like a "dark frame" subtraction (which is doable in principle).
Accuracy concerns aside, though, everything we see there (when you consider the physical context) supports the fact that the metal casing helps spreading the heat (which is obvious, it's a material which high heat conductivity, and there wouldn't be any need to put it there otherwise).
Even the 60C reading must be off by some for the same reason (given the regions appearing at around 70C), of course, but I assume OP doesn't care about that level of accuracy.
I disagree that the thermal image provides evidence of those truths
Does the image prove the CPU has a low temperature? No, the image reports the temperature inaccurately. Does the image prove the package has no hotspots? No, it wouldn't show hotspots if they were there. Does the fact the PCB gets hot tell us much? Not really, you'd expect heat to conduct from the package and balls to the PCB no matter what the package was made from.
Which is the evidence you're looking for.
Photon size wasn't used. Micron is an unofficial name for 1e-6 m, the lengths of 0.7e-6 m to 1e-3 m correspond to the wavelength of infrared radiation:
https://en.wikipedia.org/wiki/Infrared
So "those 10 micron photons" there mean "the photons of the radiation with the wavelength of 1e-6 m."
Not that it makes the measurements in question any less inaccurate, but it's very common to see these problems in temperature measurements.
To measure real metal surface temperature by IR, you have to paint that metal (ideally, with black matte paint), or apply similarly-textured sticker.